1. Field of the Invention
The present invention generally relates to transducers for the measurement of physical quantities and, more particularly, to optical sensors including an optical fiber communication link.
2. Description of the Prior Art
Many types of machinery and systems benefit from or rely upon monitoring of conditions at various locations therein or in the environment in which such systems are operated. For example, many manufacturing processes in chemical and metallurgical fields, such as semiconductor manufacturing, require precise control of temperature, pressure, position or any number of other physical conditions, often in hostile environments. Automatic control systems, such as heating and air-conditioning systems also require similar monitoring of physical conditions, often at a large number of spatially distributed locations. Whatever the application, sensor size is often critical in avoiding interference with the process being monitored. In recent years, fiber optic sensors have often been used to meet these requirements since fiber optic sensors are characteristically immune from electromagnetic interference and exhibit high accuracy, geometric flexibility, durability and small size.
Fiber optic sensors for high accuracy measurement generally rely on the sensing of a change of dimension in response to the physical condition being measured. While such sensors may have a variety of forms, they may generally be grouped into two categories: intensity-based sensors and interferometric sensors. Interferometric sensors measure differential phase changes in multipath optical geometries and provide extremely good stability and high resolution of measurement. However, complex signal processing is often required to accomplish signal recovery. Intensity-based sensors, by comparison, have somewhat lower resolution but require only simple signal processing such as for calibration and/or linearization of output light intensity (or power).
Both of these categories of fiber optic sensors return information from the transducer location as changes in intensity or power of light or other radiation generally extending from the infrared to ultraviolet wavelength range. Therefore, attenuation attributable to the fiber optic communication link, modal power distribution and variation in intensity of the input illumination are sources of potential error or drift over time of the measurement being made and may be interpreted as changes in the measurand. For this reason, considerable effort has been expended on various compensation techniques to reduce variation and drift in fiber optic measurement arrangements. However, while these techniques can provide some degree of compensation for variations of fiber loss or input power, variations in temperature of the sensor (when the sensor is used to measure other physical parameters) may cause additional variation in response.
Since this further source of measurement variation originates with the sensor or transducer, itself, the variation cannot be compensated without an independent temperature measurement, which has required a further, independent temperature measurement system. Provision of such a further system potentially compromises most of the benefits of size and installation flexibility characteristic of fiber optic sensors. Further, it may be difficult to obtain a reliable measurement of temperature of one sensor with an independent temperature sensor while maintaining the independent temperature sensor free from the physical effects which the one sensor is intended to measure.